Power conversion systems are used to generate and provide AC output power to a load, such as a single or multi-phase AC motor driven by an inverter stage of a motor drive power converter. The power converter typically receives AC input power from a three-phase supply, and an input rectifier provides a DC bus voltage from which the inverter generates AC output signals to drive the load. In voltage source converters, the DC bus or DC link includes one or more capacitors, which are often electrolytic capacitors. However, excessive ripple voltage on the DC bus increases ripple current flowing in the bus capacitor(s), which can stress electrolytic capacitors, potentially leading to overheating and shortening of capacitor operational lifetime. Early detection of excessive DC bus ripple voltage can be used to provide an alarm or safely shutdown the power converter prior to bus capacitor damage or stress. However, many ripple detection approaches involve additional circuitry or are computationally intensive, adding cost and complexity to the system. Another problem in power conversion systems is loss of an AC input phase, which can increase DC bus ripple voltage and reduce the DC voltage supplied to the inverter. Accordingly, detection of input phase loss can also be used to trigger an alarm or power converter fault for safe continued operation or shutdown. Conventional phase loss detection techniques, however, are generally either complicated and costly or unable to adequately detect all phase loss conditions. For instance, simple detection techniques have been proposed in which the difference between the DC bus voltage and the DC bus voltage squared is compared with a threshold, and if the threshold is exceeded, it is assumed that either excessive DC bus ripple is present or a phase loss has occurred. Other techniques involve detection of instantaneous DC bus voltage and comparing this to upper and lower limit boundaries, as well as calculating an average value of the ripple period in one sample cycle, but these techniques are prone to noise and suffer from low accuracy and slow response speed. Accordingly, improved techniques and apparatus are desirable for cost effective and robust detection of excessive ripple and phase loss in a power conversion system.